This invention relates to a process for oxidizing ammonia to form nitric oxide.
The oxidation of ammonia (NH.sub.3) to form nitric oxide (NO) is a reaction of great industrial significance, since the production of nitric oxide is the first step in the synthesis of nitric acid. Nitric acid currently ranks tenth among the chemicals produced in the United States with a yearly domestic production rate of 8.6 million tons. Industrially, the conversion of NH.sub.3 to NO is carried out over fine mesh platinum (90%)-rhodium (10%) alloy screens at temperatures between 750.degree. and 950.degree. C. Pressures within the range 1-9 atm are typically employed with a feed stream of 10% NH.sub.3 in air. The reaction is extremely rapid (one millisecond or less) and conversion of NH.sub.3 to NO ranges from 94-98%. The disadvantages of the Pt catalyst are its high price and operating cost. For a plant operating at 1 atm and 800.degree. C., a catalyst loss of 50 mg per ton 100% nitric acid is representative.
The oxides of Fe, Mn, Bi, Co, Ni, Cr and Cu have long been studied as alternative catalysts for the NH.sub.3 oxidation process. During World War I, a composition of 90% Fe.sub.2 O.sub.3, 5% Mn.sub.2 O.sub.3 and 5% Bi.sub.2 O.sub.3 was used as a substitute in the form of granules or pellets. Presently, a porous iron-chromium oxide catalyst deposited on a mechanically strong support is used in industry in a second stage oxidation of NH.sub.3. This catalyst can attain conversions of 97% with a 10% NH.sub.3 air mixture, contact times on the order of 20 milliseconds and temperatures in the range 700.degree.-800.degree. C.
The oxide metal catalysts suffer from several disadvantages. Their lifetime usually does not exceed three months and their mechanical strength is low. Because the non-platinum catalysts are used in the form of pellets or granules (either unsupported or supported) and because the required contact time is longer than for Pt, more cumbersome reactors are necessary for their utilization. Consequently, these substitutes cannot compete economically with Pt.